The present invention relates to hydrogen absorbing alloy electrodes for use as negative electrodes of nickel-metal hydride batteries.
While nickel-cadmium batteries and lead batteries are already in wide use as secondary batteries, it is desired especially in recent years to develop secondary batteries which have a high energy density and are yet clean with a advances in compact information devices such as cellular phones and notebook computers. Accordingly, attention has been directed to closed-type nickel-metal hydride batteries wherein an electrode of hydrogen absorbing alloy is used as the negative electrode.
Nickel-metal hydride batteries comprise a negative electrode of hydrogen absorbing alloy, a positive electrode of nickel hydroxide, an alkaline electrolyte, a separator, etc. The hydrogen absorbing alloy electrode serving as the negative electrode is prepared by adding a binder to a hydrogen absorbing alloy powder obtained by pulverizing a hydrogen absorbing alloy ingot, and forming the mixture to a shape of an electrode.
With nickel-metal hydride batteries wherein a negative electrode of hydrogen absorbing alloy is used, a gas-phase reaction and an electrochemical reaction proceed at the same time on the surface of the hydrogen absorbing alloy by virtue of the contact of the alloy surface with the alkaline electrolyte. More specifically, in the relationship between the hydrogen pressure and the temperature, hydrogen is absorbed by the hydrogen absorbing alloy, or the hydrogen absorbing alloy desorbs hydrogen (gas-phase reaction). In the relationship between the voltage and the current, on the other hand, application of voltage (charging) causes absorption of hydrogen by the hydrogen absorption alloy of the hydrogen produced by the electrolysis of water, and delivery of current (discharge) causes oxidation of hydrogen to water (electrochemical reaction).
However, the conventional nickel-metal hydride battery has the problem of failing to achieve sufficiently high high-rate discharge characteristics.
An object of the present invention is to provide a hydrogen absorbing alloy electrode which achieves higher high-rate discharge characteristics than conventionally, and a nickel-metal hydride battery using the electrode. The present inventors conducted intensive research to clarify the cause of failing to achieve sufficiently high high-rate discharge characteristics for the conventional nickel-metal hydride battery. As a result, it revealed that the above cause is that carboxymethylcellulose (CMC), etc. which is conventionally used as a binder is low in retention of liquids, to accomplish the present invention.
Specifically, with the conventional nickel-metal hydride batteries, the binder has the reduced retention of liquids since CMC is low in retention of liquids, to give reduced retention of liquids to the electrode in its entirety. Because of this a supplying speed of hydroxide ion to the negative electrode is kept low, to lead to reduced likelihood of discharge reaction. Accordingly, operating voltage at the time of high-rate discharge is decreased, failing to achieve sufficiently high high-rate discharge characteristics.
The hydrogen absorbing alloy electrode of the invention is prepared by adding a binder to a hydrogen absorbing alloy powder, and forming the mixture to a shape of an electrode. The binder is partly or entirely made of poly N-vinyl acetamide (PNVA). With the hydrogen absorbing alloy electrode of the invention, PNVA for use as the binder is higher in retention of liquids than CMC used conventionally, increasing an amount of retention of liquids by virtue of the binder, to increase an amount of retention of liquids for the electrode in its entirety. The increase of the retention of liquids in amount for the electrode in its entirety leads to the high supplying speed of hydroxide ion to the negative electrode, to activate the discharge reaction. Consequently, operating voltage is high at the time of high-rate discharge to improve the high-rate discharge characteristics.
Stated specifically, said PNVA is added in a proportion of 0.1 to 1.0 wt. % based on the hydrogen absorbing alloy powder. In the case where PNVA is added in a proportion below 0.1 wt. % based on the hydrogen absorbing alloy powder, the electrode in its entirety is not effectively increased in the amount of retention of liquids by virtue of PNVA, so that high-rate discharge characteristics can not be improved sufficiently. On the other hand, when PNVA is added in a proportion greater than 1.0 wt. % based on the hydrogen absorbing alloy powder, the contact between alloy particles and that between a substrate and an alloy particle will be deteriorated by the increase of the amount of the binder, to increase the resistance of the electrode, having an operating voltage at the time of high-rate discharge decreased, failing to achieve sufficiently high high-rate discharge characteristics. Consequently, PNVA is preferably added within the range of the above.
Further specifically, the binder is partly made of water-soluble macromolecule and/or rubber resin. The water-soluble macromolecule or the rubber resin is relatively excellent in binding materials, to enhance binding force between the substrate and the alloy particles, retaining an alloy particle on the substrate reliably. The water-soluble macromolecule or the rubber resin has a relatively high elasticity, to prevent a hydrogen absorbing alloy from dropping from an electrode when a separator is interposed between a hydrogen absorbing alloy electrode serving as a negative electrode and a positive electrode to have them rolled up.
Stated more specifically, the water-soluble macromolecule and the rubber resin are added in a combined proportion of 0.2 wt. % to 2.0 wt. %, or the water-soluble macromolecule or the rubber resin is added in a proportion of 0.2 wt. % to 2.0 wt. % based on the hydrogen absorbing alloy powder. In the case where the water-soluble macromolecule and the rubber resin are added in a combined proportion below 0.2 wt. %, or the water-soluble macromolecule or the rubber resin is added in a proportion below 0.2 wt. % based on the hydrogen absorbing alloy powder, binding force between alloy particles and that between a substrate and an alloy particle is weakened, to deteriorate the contact between them, increasing the resistance of the electrode relatively, to have an operating voltage at the time of high-rate discharge relatively decreased, failing to achieve sufficiently high high-rate discharge characteristics. On the other hand, in the case where the water-soluble macromolecule and the rubber resin are added in a combined proportion greater than 2.0 wt. %, or the water-soluble macromolecule or the rubber resin is added in a proportion greater than 2.0 wt. % based on the hydrogen absorbing alloy powder, binding force between alloy particles and that between a substrate and an alloy particle is deteriorated by the increase of the amount of the binder, to increase the resistance of the electrode, having an operating voltage at the time of a high-rate discharge decreased, failing to achieve sufficiently high high-rate discharge characteristics. Accordingly, the water-soluble macromolecule and the rubber resin are preferably added in a combined proportion within the range of the above, or the water-soluble macromolecule or the rubber resin is preferably added in a proportion within the range of the above.
Further more specifically, the water-soluble macromolecule is polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA) or hydroxy propyl cellulose (HPC), and the rubber resin is acrylic acid ester, polystyrene, silicone, or a copolymer of styrene-methacrylic acid ester-acrylic acid ester. The binder described is partly made of one or more kinds of the water-soluble macromolecule and/or rubber resin selected from among the water-soluble macromolecule or the rubber resin.
The hydrogen absorbing alloy electrode is prepared by adding a binder to the hydrogen absorbing alloy powder to obtain a paste, coating a surface of a substrate (a current collector) with the paste, and drying the paste. With the specific construction, PNVA for use as a binder, and the water-soluble macromolecule and the rubber resin are higher in heat-resistance than CMC used conventionally, so that the paste can be dried at a temperature of 100xc2x0 C. to 150xc2x0 C. Accordingly, while the paste is heretofore dried at around 50xc2x0 C. due to CMC having a lower heat-resistance, the drying can be made within a shorter period of time than conventionally. As a result, the electrode can be prepared in a shorter period of time.
As described above, with the nickel-metal hydride battery using the hydrogen absorbing alloy electrode of the invention, higher high-rate discharge characteristics can be obtained than conventionally.